Can't I use services like Sendgrid or Mandrill directly?

All email services require some sort of authentication to send the emails on your behalf. That makes it a bad idea to use them directly from client-side – revealing your password or your secret keys will allow anyone to send emails on your behalf.

EmailJS keeps your authentication details on the server-side, and the client-side code just triggers a predefined email template, similarly to how any client-server application is working.